The present invention is directed to an improved fluid mount for isolating a source of vibration, such as an aircraft engine or the like, from its support structure. More particularly, the present invention is an improvement, for certain applications, over the fluid mount described in U.S. Pat. No. 4,811,919, which is hereby incorporated by reference.
As described in that commonly assigned patent and as shown in FIGS. 1, 2 and 3 thereof, a fluid mount is used to isolate the forward bulkhead 12 of engine E from beam 11 which is attached to pylon P. An external tube 45 interconnects a pair of fluid chambers and serves as an inertia track, attenuating vibrational energies for a particular design frequency by means of fluid inertia. As the '919 patent indicates, different frequencies can be attenuated by changing tube 45, i.e., by altering the length (L) to area (A) ratio of the inertia track.
The problems with the system depicted in '919, for applications where inertia track changeout is desirable, are twofold. First, although changing the tube has been simplified, the fact that changeout is necessary creates difficulties. In an engine test environment where time is money, having to break down the system, change tube 45 for one having a different length to area ratio, repressurize the fluid, carefully bleed all air from the fluid chambers to enable a second test to be run to determine which tube L/A ratio provides better overall performance, costs a significant amount more than is reflected by the hydraulic fluid which is lost during changeout. Such a changeout will also be an expensive proposition should it be desirable to replace inertia track 45 once the mount has been installed on an aircraft since any down time for such maintenance means taking the plane out of operation.
Second, since in actual operation, only a single inertia track can be used, a single notch frequency can be provided to cope with vibrational frequencies which is tuned for only one mode of operation. The troublesome frequencies during takeoff, for example, will, for most engines, be significantly different than the vibrational frequencies during normal cruise operations. Accordingly, tuning the fluid mount to isolate vibration in one operational mode will sacrifice some performance for most, if not all, other modes of operation.
It is among the purposes of the present invention to overcome the difficulties associated with this type of fluid mount. In a first preferred embodiment, a digitally adaptive mount is provided by an adjustable member for interconnecting first and second fluid chambers of a double pumper mount each of which has an ingress/egress port, in which the adjustable member has a plurality of fluid inertia tracks, each with a first terminal end portion, which may be selectively interconnected with the first ingress/egress port and a second terminal end portion simultaneously connectable with the second ingress/egress port. Preferably, these tracks are provided as helical passageways or grooves cut spirally about an external surface of the cylindrical adjustable member, each passageway providing a different notch frequency resulting from its unique L/A ratio. In a test environment, the cylindrical adjustable member may be adjusted to and locked in one of its three positions for a first engine test. Subsequent second and third test runs can be made without the need to break down the system into its components, thereby avoiding the need to depressurize/repressurize the hydraulic system. In actual use with an aircraft, the plurality of inertia tracks can each be designed with an L/A ratio to optimize the performance parameters for a particular operating condition. A simple diagnostic/control circuit can be provided along with a simple motor to adjust the mount to provide optimum performance in response to the conditions sensed by the circuit.
In a second embodiment, a continuously adaptive inertia track is provided. First and second at least partially annular grooves, any portion of which can interface with the two ingress/egress ports of the two fluid chambers, are formed on the circumference of the cylindrical adjustable member and are interconnected by a single helical groove. By rotating the cylindrical member through some portion of up to about 270.degree., the effective length of the inertia track is adjusted by adding or subtracting a portion of the length of the first partially annular groove, up to the 270.degree. limit, to and from the length of the helical groove. The second annular groove is intentionally large to minimize its inertial effects from the system. This second groove is of such a size that the fluid system views it as being a portion of the second fluid chamber.
Various other features, characteristics and objectives of the present invention will become apparent after a reading of the following detailed description of the invention.